1. Field of the Invention
This invention relates to a novel and improved catalyst which is effective for the treatment of a heavy hydrocarbon oil containing asphaltenes, especially for the decomposition and conversion of asphaltenes into lower molecular compounds, and the removal of metals and sulfur from asphaltenes. This invention is also directed to a method of preparing such a catalyst and to a process for hydrotreating an asphaltene-containing heavy hydrocarbon oil using such a catalyst.
2. Description of the Prior Art
The heavy hydrocarbon oils to which this invention is applicable include reduced crude oils, vacuum residues, certain crude oils produced in South America, etc., heavy oils extracted from tar sand or oil sand produced in Canada, etc., and mixtures thereof. These hydrocarbon oils usually contain asphaltenes, heavy metals, sulfur compounds, nitrogen compounds, or the like. The term "asphaltene" used herein means a substance which is insoluble in normal heptane (n-heptane), and mostly composed of high molecular condensed aromatic compounds. These compounds are associated with one another to form micellar colloids in heavy oils. Specific examples of such heavy hydrocarbon oils include high-asphaltene and high-heavy metal Venezuelan crude oil having a specific gravity (.ANG.PI) of 9.4, an asphaltene content of 11.8% by weight, a vanadium content of 1,240 ppm, a sulfur content of 5.36% by weight and a nitrogen content of 5,800 ppm, atmospheric residues from Canadian tar sand extracted oil and having a specific gravity (.ANG.PI) of 9.2, an asphaltene content of 8.1% by weight, a vanadium content of 182 ppm, a sulfur content of 4.41% by weight and a nitrogen content of 4,200 ppm, and vacuum residue of Middle and Near East oil and having a specific gravity (.ANG.PI) of 5.1, an asphaltene content of 14.6% by weight, a vanadium content of 165 ppm, a sulfur content of 5.24% by weight and a nitrogen content of 4,000 ppm.
Table 1 shows the properties of typical heavy hydrocarbon oils. In the table, the letters A to F indicate the following oils, respectively:
______________________________________ A: Boscan crude oil B: Athabasca bitumen C: Khafji vacuum residue D: Gach Saran vacuum residue E: Kuwait vacuum residue F: Gach Saran atmospheric residue ______________________________________
TABLE 1 ______________________________________ Properties of feedstock oils A B C D E F ______________________________________ Specific 9.4 9.2 5.1 4.8 6.0 16.4 gravity, API Carbon, wt % 83.06 83.11 83.11 84.85 83.42 85.35 Hydrogen, wt % 10.49 10.50 10.05 10.36 10.12 11.50 Sulfur, wt % 5.36 4.41 5.24 3.67 5.25 2.62 Nitrogen, wt % 0.58 0.42 0.40 0.65 0.42 0.36 Conradson carbon 15.8 13.5 23.8 21.6 23.0 8.88 residue, wt % Asphaltenes, wt % 11.8 8.1 14.6 7.8 4.9 2.87 Metals, wt ppm Ni 106 79 53 92 35 42 V 1,240 182 165 298 117 130 ______________________________________
As shown above, a heavy hydrocarbon oil contains a very large amount of impurities, such as sulfur and nitrogen compounds, vanadium and nickel. Such impurities are contained in the asphaltene fraction in concentrated state, and make catalytic hydrodesulfurization difficult. Heavy hydrocarbon oils having such a high content of asphaltenes exist abundantly in nature, and while they are considered as promising hydrocarbon resources, they are presently used merely for producing extremely low grade fuel oil or asphalt for pavement of roads. When these heavy hydrocarbon oils are used as fuel and burnt, they produce oxides of sulfur, nitrogen, heavy metal, etc. which cause air pollution. Despite these disadvantages, heavy hydrocarbon oils containing asphaltenes and heavy metals are important under the present political and economical situation facing energy crisis due to the depletion of high quality petroleum resources in the near future. It is strongly desired to develop technology which is effective for converting those heavy hydrocarbon oils to more useful hydrocarbon oils containing no substance causing environmental pollution, and which are substantially free from any asphaltenes or heavy metals.
Various kinds of catalysts and desulfurization processes have been proposed for hydrodesulfurization of a heavy hydrocarbon oil having a relatively low asphaltene and heavy metal content to obtain a higher grade desulfurized oil, and some of them have already been used commercially. A typical process employs a fixed or ebullated bed by which a heavy hydrocarbon oil is hydrodesulfurized directly. The development of this direct hydrodesulfurization process is largely attributable to the improved catalyst performance [M. W. Ranney, Chemical Technology Review No. 54, "Desulfurization of Petroleum", Noyes Data Corporation, New Jersey (1975)]. It is, however, well known among those of ordinary skill in the art of petroleum refining that a number of economical disadvantages may result from the use of this process if the oil to be treated contains large amounts of asphaltenes and heavy metals, because the macromolecules of asphaltenes are colloidally dispersed in the oil and are not able to diffuse easily into the active sites in the pores of the catalyst. This seriously inhibits the hydrocracking of asphaltenes, and the presence of the asphaltenes inhibits desulfurization and other reactions for hydrotreating the hydrocarbon oil. Another obstacle to the practical application of the direct hydrodesulfurization process lies in the formation of coke and carbonaceous materials highly promoted by the presence of asphaltenes, leading to a sharp reduction in the activity of the catalyst. The formation of such carbonaceous materials does not only occur in the intraparticles of the catalyst, but also in the interparticles among the catalyst particles. If the feedstock oil contains a large amount of asphaltenes, an increased amount of carbonaceous material derived from asphaltenes is deposited into the spaces among the catalyst particles, and the gummy carbonaceous sediment unites the catalyst particles together. This causes blocking of the catalyst particles and plugging of the catalyst bed, so that there occur serious problems, such as maldistribution of the reactant flow through the bed, and an increased differential pressure across the bed.
A further serious disadvantage of the direct hydrodesulfurization process resides in an extremely shortened catalyst life which is due to the poisoning and pore-plugging action of the metals contained abundantly in the feedstock oil, namely due to the metal deposition on the catalyst active surfaces.
The catalytic hydrotreatment of heavy hydrocarbon oils, using the conventional catalyst, requires an extremely high catalyst consumption relative to the amount of the oil being handled, and even if the aforementioned disadvantages may have been overcome, the conventional catalysts make it imperative to set severe conditions for the reactions, which accelerate catalyst deactivation, in the event the operation is primarily intended for decomposing asphaltenes selectively to obtain light oil. Moreover, a high rate of gasification resulting from the secondary cracking of light oil prevents production of light oil at a high yield, and an increased hydrogen consumption causes a serious problem in the economy of the operation. It has also been pointed out that the gummy matter contained in the product oil lowers its thermal stability, and that sludge material is likely to settle down, causing phase separation (U.S. Pat. No. 3,998,722). Accordingly, in order to obtain a low sulfur fuel oil by hydrodesulfurization of a heavy hydrocarbon oil having high asphaltene and heavy metal contents, e.g., containing at least about 5% by weight of asphaltenes and at least about 80 ppm of vanadium, pretreatment of the heavy hydrocarbon oil has currently been required [C. T. Douwes, J. Van Klinken et al., 10th World Petroleum Congress, Bucharest, 1979, PD-18(3)]. A variety of processes have been proposed for the pretreatment, and can be classified into the following two groups. One of the groups covers the processes for removing asphaltenes and heavy metals from the feedstock oil by extraction, such as solvent deasphalting or otherwise physically, or by heat treating, such as coking. These methods, however, produce a considerably large quantity of by-products, such as deasphalting residue (asphalt) or coke, and the effective utilization of these heavy by-products is not feasible, since they contain highly concentrated impurities, such as heavy metals, sulfur and nitrogen. As the quantity of the by-products increases with an increase in the asphaltene content of the feedstock oil, the application of these processes is not effective.
Another group involves hydrotreatment of a heavy hydrocarbon oil in the presence of an appropriate catalyst, mainly for the hydrodemetallization thereof to thereby reduce the poisoning to the catalyst by heavy metals in the subsequent catalytic hydroprocessing. There have been proposed various types of catalysts and processes for such pretreatments. For example, there are known demetallization processes using inexpensive catalysts, such as natural minerals containing alumina, such as bauxite, ores such as manganese nodules and nickel ore, and industrial waste such as red mud and spent desulfurization catalysts (U.S. Pat. Nos. 2,687,985, 1,769,758, 2,771,401, 3,839,187, 3,876,523, 3,891,541 and 3,931,052 and Japanese Laid-Open Patent Applications Nos. 13236/1972, 21688/1973, 5402/1974, 121805/1974, 122501/1974, 78203/1978 and 3481/1979). These catalysts are, however, unsatisfactory. Some of them have certainly high activity for demetallization, but still encounter the problem of deactivation by metal deposition because they have a small pore diameter or an insufficient pore volume. Other catalysts have an insufficient surface area resulting in an insufficient activity for demetallization, thereby requiring the treatment to be carried out in a relatively high temperature operation. Consequently, carbonaceous matter is formed by polycondensation, etc. of high molecular compounds such as asphaltenes, and the catalysts are strongly deactivated by coke. Thus, these pretreatment methods are faced with a number of problems, such as reduction in the activity of the demetallization catalyst during continuous operation, and necessity of the regeneration or disposal of the spent catalyst. Although demetallization with such catalysts may to some extent reduce the poisoning of the catalyst by heavy metals in the subsequent desulfurization processing, it hardly serves to remove macromolecules of asphaltenes from a heavy oil, and leaves outstanding problems of catalyst poisoning, plugging, or the like by asphaltenes.
There have been many proposals made for improving the demetallization activity by specifying the kind and quantity of the catalytic metal component for hydrotreatment to be used in the catalyst which is similar to those for the hydrodesulfurization of heavy hydrocarbon oils (see, e.g., U.S. Pat. Nos. 2,577,823, 2,730,487, 2,764,525, 2,843,552, 3,114,701, 3,162,596, 3,168,461, 3,180,820, 3,265,615, 3,297,588, 3,649,526, 3,668,116, 3,712,861, 3,814,683, 3,876,680, 3,931,052, 3,956,105 and 3,960,712, and Japanese Patent Publications Nos. 20914/1971, 33223/1971 and 9664/1974). There are, however, problems in the practical application of these catalysts for the hydrodemetallization of heavy hydrocarbon oils having high asphaltene and heavy metal contents, since various difficulties, such as the poisoning of the catalyst by asphaltenes and heavy metals, still remain substantially unsolved as pointed out with reference to the previously-described hydrotreating catalyst. Various hydrotreating processes have been proposed for the purpose of overcoming these problems (U.S. Pat. Nos. 1,051,341, 2,890,162, 3,180,820, 3,245,919, 3,340,180, 3,383,301, 3,393,148, 3,630,888, 3,640,817, 3,684,688, 3,730,879, 3,764,565, 3,876,523, 3,898,155, 3,931,052, 3,902,991, 3,957,622, 3,977,961, 3,980,552, 3,985,684, 3,989,645, 3,993,598, 3,993,599, 3,928,176, 3,993,601, 4,016,067, 4,054,508, 4,069,139 and 4,102,822, Japanese Patent Publications Nos. 38146/1970, 18535/1972, 17443/1973, 16522/1974, 18763/1974, 18764/1974, 3081/1975, 26563/1979, and Japanese Laid-Open Patent Applications Nos. 2933/1971, 5685/1972, 44004/1974, 121805/1974, 123588/1975, 31947/1973, 9664/1974, 144702/1975, 160188/1975, 4093/1976, 55791/1976, 30282/1977, 50637/1977, 22181/1978, 23303/1978, 145410/1977, 36485/1978, 2991/1979, 11908/1979, 14393/1979, 23096/1979, 104493/1979, 112902/1979 and 125192/1979).
The aforementioned processes proposed for attaining this purpose can be classified roughly into the following groups with respect to the catalyst to be used:
(1) Process for hydrotreating characterized by using a catalyst having small pores (i.e., catalyst having a peak of a pore diameter at about 100 .ANG. or below in its a pore volume distribution);
(2) Process for hydrodesulfurization and demetallization characterized by using a catalyst having middle pores (i.e., catalyst having a pore volume which is for the greater part occupied by pores having a diameter in the range of about 100 .ANG. to 200 .ANG.);
(3) Process for hydrodemetallization characterized by using a catalyst having macro pores (i.e., catalyst having a pore volume of which the greater part is occupied by pores having a diameter of about 200 .ANG. or above);
(4) Process for hydrodemetallization and desulfurization characterized by using a catalyst having both the pore characteristics mentioned at (2) and (3) above;
(5) Process for hydrodesulfurization and demetallization characterized by using a catalyst having a double-peak in its pore volume distribution defined by both the pore characteristics mentioned at (1) and (3) above;
(6) Process for multistage hydrotreatment with a combination of some treatments using the aforementioned catalysts;
(7) Process for hydrotreating using a catalyst comprising a a composition which is substantially the same as one of the aforementioned catalysts, but having a specific shape; and
(8) Process for hydrotreating characterized by the mode and conditions of the reaction.
None of the processes listed above is, however, satisfactory, since none of them gives a basic solution to the aforementioned technical problems found in the hydrotreatment of heavy hydrocarbon oils containing large amounts of asphaltenes and heavy metals. The problems involved in each of these proposed processes will hereunder be pointed out, together with its characteristic features.
The process belonging to the group (1) is intended for coping with the difficulties which are due to the metal compounds present in heavy hydrocarbon oils, and uses a catalyst having a narrow pore volume distribution defined by small pores which are capable of excluding macromolecules of asphaltenes. According to this method, therefore, asphaltenes are hardly demetallized or desulfurized, but metal and coke are likely to deposit near inlets of pores. Because of these disadvantages, this process is unsuitable for the treatment of a heavy hydrocarbon oil containing large amounts of asphaltenes and heavy metals, and is applicable only to oil having a heavy metal content of about 50 ppm or below.
The process of the group (2) is widely utilized on a commercial scale for the hydrodesulfurization of an atmospheric residue, and makes it possible to lessen to some extent the reduction in the activity of the catalyst which is due to the heavy metal and asphaltenes in the feedstock oil. However, the effect of this method depends mainly on the metal content of the feedstock oil, and the application of the process is substantially limited to oil having a metal content of about 80 to 100 ppm, or below. The diameters of the pores in the catalyst used for this process are sharply reduced at the mouths thereof by deposition of metals therein, and the diffusion of macromolecules of asphaltenes or the like into the pores is greatly inhibited. Therefore, there are necessarily limitations to the asphaltene content of the feedstock oil which can be treated by this method, and the application of this method is limited to a feedstock oil having an asphaltene content of about 5% by weight or below.
The process of the group (3) uses a catalyst of which nearly all the pores have a diameter of at least about 200 .ANG. in order to facilitate diffusion of metal compounds into the pores. The increased pore diameter facilitates the diffusion into the pores of high molecular compounds having a high heavy metal content, but as the catalyst has a sharply reduced pore surface area, its activity for demetallization is not substantially improved. If the volume of macropores is increased to enlarge the pore surface area, coke precursors, such as asphaltenes, stay in the pores for a long time, thereby promoting the coking reaction, with a resultant increase in the deposition of coke in the catalyst pores. Thus, most of the catalysts employed for carrying out this method have a relatively low activity for demetallization. If for hydrotreating, the conditions for the reaction are made severer in order to raise the demetallization activity of the catalyst to a practically acceptable level, and particularly if the reaction temperature is increased, there occur an excess consumption of hydrogen, and other problems which are similar to those already pointed out with respect to the process in which inexpensive material, such as bauxite, is used as a catalyst. In the event a macropore catalyst with many pores having a diameter of at least about 400 .ANG. is used, a slight increase in the surface area does not improve the performance of the catalyst very much, since the amount of the coke to be deposited also increases. Further drawbacks which are common to such a molded catalyst include an insufficient mechanical strength which is likely to cause disintegration of the catalyst due to its breakage and abrasion when it is charged into the reactor, and during the operation. According to this process, the catalyst is primarily intended for demetallization, and does not have any appreciable effect on the decomposition of asphaltenes.
The process of the group (4) is an improvement over the processes (2) and (3), and uses a catalyst having specifically controlled ranges of pore volume and particle diameter in a pore diameter range of at least 100 .ANG.. This process is, however, primarily intended for demetallization of the feedstock oil as a whole, or simultaneous demetallization and desulfurization, and is not considered complete for either demetallization or desulfurization. The process is not considered to provide a very satisfactory improvement from the standpoint of efficient use of the catalyst, either.
Generally, a heavy hydrocarbon oil contains large amounts of impurities, such as heavy metals and sulfur compounds, which are widely distributed, not only in high molecular fractions such as asphaltene fractions, but usually considerably in relatively low molecular hydrocarbon oil fractions as well. It is practically difficult to obtain a catalyst having pores with a diameter range which is suitable for the demetallization and desulfurization of both asphaltene and low molecular fractions, and a pore surface area which is sufficiently large to satisfy both of the purposes. It is actually impossible to obtain a single catalyst which is capable of performing the functions of demetallization and desulfurization simultaneously to an optimum degree. The molecular sizes of the heavy metal and sulfur compounds which a heavy hydrocarbon oil contains differ over a wide range, and their diffusion into the catalyst pores has largely different effects on the reactions of each molecule. The rates of desulfurization and demetallization of asphaltenes are extremely low as compared with those of other light oil fractions. Moreover, a comparison between the demetallization and desulfurization rates reveals that demetallization is more likely to be affected by the intrapore diffusion. It is practically impossible to accomplish with a single catalyst the hydrodemetallization or desulfurization of a heavy hydrocarbon oil containing large amounts of asphaltenes and heavy metals at a substantially satisfactory reaction rate, since it means an excessive need for the total surface area and total pore volume of the catalyst. The surface area of a catalyst is determined almost solely by the pore diameter and pore volume, and an increase in the pore diameter results in a sharp reduction of the surface area, as already pointed out.
Thus, the maximum pore volume of the catalyst for the group (4) is inevitably limited to the level which the mechanical strength required thereof permits, and the catalyst fails to show any such degree of activity as the optimum catalysts for the groups (2) and (3) can provide individually for desulfurization or demetallization. Those portions of the catalyst which are provided with pores having a diameter of about 100 to 200 .ANG. lose their catalytic activity rapidly due to pore plugging by metal accumulation, while the remaining portions with larger pores has only a limited surface area and do not contribute to the reaction. The catalyst for the group (4) does not provide any appreciably improved efficiency, since it does not always permit a sufficiently large amount of metal to be accumulated thereon before the rates of the various reactions involved can be kept equal, as opposed to the optimum catalysts for the groups (2) and (3). In order to overcome these disadvantages, it is necessary to use a catalyst composed of very fine particles, but such a catalyst is not suitable for a fixed or ebullated bed system which is usually employed for the hydrotreatment of a heavy hydrocarbon oil. As the demetallization of heavy hydrocarbon oils is generally determined by the rate of intrapore diffusion, the overall reaction velocity shows a decrease as an exponential function of the diameter of the catalyst particles.
The process of the group (5) is based on the fact that in the hydrotreatment of heavy hydrocarbon oils, desulfurization is not very largely influenced by intrapore diffusion, while demetallization is largely affected by it. According to this process, there is used a catalyst provided with both small pores having a diameter not greater than about 100 .ANG., and macropores having a diameter of at least about 500 .ANG., or even at least about 1,000 .ANG.. Although this catalyst does certainly relax the limitations relating to the diffusion of metal-containing high molecular compounds into the pores, it shows a sharp reduction in activity due to metal accumulation in the pores having a diameter not greater than about 100 .ANG., and the mouths of these pores are likely to be blocked, as is the case with the catalyst used for the group (1). Thus, the catalyst for the group (5) fails to maintain a high activity for a long time for the feedstock oil having a high metal content, and eventually, only the larger pores act mainly for demetallization. Therefore, it is not considered to have an improved efficiency over the catalysts for the groups (1) and (3) which are used individually.
Groups (6), (7) and (8) have been proposed to improve the efficiency of hydrotreating of heavy hydrocarbon oils by selecting a catalyst composition from the groups (1) to (5), but fail to provide any basic solution to the problems inherent in the catalysts for the groups (1) to (5).
It will be noted from the foregoing that the various proposals made for the hydrotreatment of heavy hydrocarbon oils share a number of disadvantages. First of all, there has been no proposal suggesting an optimum catalyst for the decomposition of asphaltenes. Moreover, none of the processes proposed for simultaneous desulfurization and demetallization with a single type of catalyst is well aware of the fact that the great difference in molecular size between asphaltene and the other oil fractions leads to a large difference in the reaction velocity between desulfurization and demetallization.
As already pointed out repeatedly, the pretreatment of oil for effective reduction and removal of asphaltenes therefrom is essential for obtaining a high grade hydrocarbon oil by hydrotreating the feedstock oil containing a large amount of asphaltenes. There has been proposed no catalyst that is suitable for that purpose. A number of attempts have, however, been made recently for decomposing asphaltenes in heavy hydrocarbon oils. For Example, various processes have been proposed in Japanese Patent Publications Nos. 33563/1976, 42804/1977 and 5212/1978 which have recently been issued. These processes propose conversion of a heavy hydrocarbon oil to a light hydrocarbon oil by dispersing vanadium sulfide, e.g., vanadium tetrasulfide, in the heavy hydrocarbon oil to form a slurry mixture, or mixing oil-soluble vanadium, e.g., vanadium resinate, into the heavy oil, and activating the vanadium at high temperature and hydrogen pressure, so that fine particles of the activated vanadium sulfide may be circulated for use as the catalyst. In order to obtain a satisfactory rate of decomposition for asphaltenes, however, it is necessary to raise the reaction temperature, or increase the concentration of the catalyst. Moreover, it can easily be supposed that these processes may reveal new and serious disadvantages when put into practice, since they all involve a slurry process in which a vanadium sulfide catalyst having no carrier is used. A typical slurry process for catalytic hydrotreatment at high temperature and pressure has long been known as a process for direct liquefaction of coal. It is known that these slurry processes share a number of drawbacks which must be eliminated before they can be adopted on an industrial basis. For example, the operation is complicated, troubles, such as blocking of the passage, are likely to occur, and special technical consideration must be given to the recovery of the fine-grained catalyst from the apparatus used and the heavy fractions.
As pointed out, it is difficult to achieve catalytic hydrotreatment of a heavy hydrocarbon oil containing large quantities of asphaltenes and heavy metals, such as vanadium, by any conventional process in a fixed bed or other reaction apparatus which is often used in industry. It is desired to develop a catalyst conforming to the requirement, and which can maintain a high activity for a long period of time.
The members of the group to which the present inventors belong became aware of the possibility that the establishment of an effective process for decomposing asphaltenes might be a key to the development of a process which would make it possible to obtain a high grade hydrocarbon oil by hydrotreating a heavy hydrocarbon oil containing large amounts of asphaltenes and heavy metals. They have continued extensive research for several years in order to develop a catalyst which eliminates the aforementioned drawbacks of the catalysts known in the art, and which is effective for the catalytic hydrotreatment of such heavy hydrocarbon oils. As a result, they have found that a catalyst composed of a naturally available clay mineral having a double-chain structure, such as sepiolite, shows a relatively high activity for the hydrotreatment of a heavy hydrocarbon oil, particularly for the decomposition and demetallization of asphaltenes, and have already proposed such a catalyst in U.S. Pat. Nos. 4,152,250 and 4,166,026, Japanese Laid-Open Patent Applications Nos. 95598/1977 and 1306/1979, etc. Further, they have paid their attention to the characteristics of heavy hydrocarbon oils containing a large amount of asphaltenes, particularly asphaltenes per se, and conducted various kinds of analyses and detailed studies for various types of oil in order to ascertain the form in which asphaltenes exist in the oils. They have consequently found that different types of asphaltenes present in different types of oil share a number of characteristics, though the asphaltene contents differ from one kind of heavy hydrocarbon oil to another. Table 2 shows the results of detailed analysis for the asphaltene fractions (a) and deasphalted fractions (b) which were obtained by removing asphaltenes from typical types of heavy hydrocarbon oils. In Table 2, the figures in parentheses indicate the weight percentages of the respective substances relative to their contents in the feedstock oil, and the letters A to E refer to the various types of feedstock oil shown in Table 1. The average molecular weight shown in Table 2 was determined by the vapor pressure osmosis method using pyridine as a solvent.
As is obvious from the results shown in Table 2, all types of asphaltenes have a lower hydrogen/carbon atom ratio than the corresponding deasphalted oils, and comprise macromolecules containing large quantities of undesirable impurities, such as sulfur, nitrogen, vanadium and nickel. The heavy metals, such as vanadium and nickel, in heavy hydrocarbon oils occupy higher proportions in asphaltenes than sulfur or nitrogen does. Asphaltenes have an average molecular weight of about 4,000 to 6,000 which indicates that they comprise macromolecules, but they do not differ very largely from one type of heavy hydrocarbon oil to another. The deasphalted oils have a higher hydrogen/carbon atom ratio than asphaltenes, extremely lower vanadium and nickel contents, and mostly an average molecular weight which is less than 1,000. It is, however, noted that at least about 60 to 80% of the sulfur and nitrogen contained in heavy hydrocarbon oils, and generally about 40 to 50% and, in some cases, over 80% of the vanadium and nickel are present in the deasphalted oils.
TABLE 2 __________________________________________________________________________ Properties of (a) asphaltenes in feedstock oil and (b) deasphalted oil A B C D E a b a b a b a b a b __________________________________________________________________________ Yield wt % 11.8 87.2 8.1 89.9 14.6 83.4 7.8 91.0 4.9 92.2 H/C 1.15 1.59 1.18 1.52 1.10 1.48 1.08 1.48 1.11 1.48 atom ratio Sulfur wt % 6.76 5.10 8.40 4.01 7.54 4.75 5.70 3.46 7.26 4.80 (14.9) (83.0) (15.4) (81.7) (21.0) (75.6) (12.1) (85.8) (6.76) (84.3) Nitrogen wt % 1.60 0.40 1.53 0.36 0.81 0.31 1.18 0.59 0.87 0.41 (32.6) (60.1) (29.5) (77.1) (29.6) (64.6) (14.2) (82.6) (10.2) (90.0) Metals wt ppm Ni 466 58 349 43 193 28 392 66 182 32 (51.9) (47.7) (35.8) (48.9) (53.2) (44.1) (33.2) (65.3) (25.5) (84.3) V 5390 594 779 109 608 90 1378 205 562 107 (51.3) (41.8) (34.7) (53.8) (53.8) (45.5) (36.1) (62.6) (23.5) (84.3) Average 5625 690 5460 694 5280 851 4730 940 4302 850 mol wt __________________________________________________________________________
The molecular weight distributions of the asphaltenes and deasphalted oils were determined by gel permeation chromatography using polystyrene gel as a molecular weight calibrating standard. The results are shown in FIG. 1. In FIG. 1, the results obtained with respect to asphaltene are shown at (a), and those relating to deasphalted oil at (b). Both in FIGS. 1(a) and (b), the axis of abscissa represents the molecular weights of those substances calibrated with polystyrene, and the axis of ordinate indicates the differential refractive indices showing the weight proportions of those substances in relation to their molecular weights. As is obvious from FIG. 1, asphaltenes and deasphalted oils have widely different molecular weight distributions from each other; asphaltenes are formed from high molecular compounds having different molecular weights ranging from about 1,000 to about 50,000, while deasphalted oil comprises compounds of which nearly all have a molecular weight in the vicinity of 1,000. Moreover, it should be noted that the molecular weight distribution of asphaltenes does not largely depend on the type of the heavy hydrocarbon oil.
In short, it is noted that a heavy hydrocarbon oil is composed of asphaltenes forming macromolecules containing large quantities of undesirable sulfur, vanadium and nickel, and an oil fraction containing compounds mostly having a molecular weight not greater than 1,000, and considered to be highly reactive for desulfurization and demetallization, and that the average molecular weight of asphaltenes and its distribution hardly change from one type of oil to another, though its amount may differ with oils. Thus, it is recognized from the results of the analyses that in order to produce a high grade hydrocarbon oil by catalytic hydrotreatment of a heavy hydrocarbon oil, it is more effective to preliminarily hydrotreat the feedstock oil with a catalyst having a porous structure best suited for the asphaltene molecules, and a high degree of selectivity and activity for the decomposition and conversion of asphaltenes into lower molecular compounds, and the demetallization or desulfurization of asphaltene molecules, and then hydrotreat the pretreated oil with another catalyst having a high activity for the desulfurization and demetallization of the oil fraction, than to hydrotreat the oil with a single catalyst.